X-ray mammography based on film-screen or xeroradiographic detection is commonly accepted as a mass screening technique for breast disease. However, certain risks are associated with x-ray examination because x-ray radiation is also ionizing. The possibility of genetic damage and radiation-induced cancer limits the recommended age group to older women as well as the frequency of exams.
More recently, broad beam light sources (sometimes referred to as "light torches") with a wide spectral bandwidth in the visible and infrared range have been used for breast imaging. The broad beam transmitted through the breast is usually recorded by a video camera, converted to an analog signal and viewed on a video monitor, or is digitized and analyzed on a computer. The ability to discriminate between various tissue-types in the breast, however, is reduced if the transmitted beam has a wide spectral bandwidth. Lesions which absorb, transmit, scatter, or reflect light to different degrees in comparison with normal tissue may exhibit reduced contrast. Moreover, the transmitted beam measurement includes the combined effects of absorption, reflection, and scattering. If a structure is highly reflective rather than strongly absorptive, the transmission measurement will not identify that property. It is thus very difficult to obtain information about the nature of the object, i.e., whether the object is a cyst, a tumor, a calcium deposit, etc.
Spatial resolution and contrast is also lost because a large amount of scattered light is transmitted from the breast to the detector. Also, structures nearest to the exit surface will cast more distinct shadows than objects close to the entrance surface. Lesion sizes that are detectable with this approach have generally been no smaller than what the physician can detect by palpation. Resolution is far below that which can be obtained with x-ray imaging systems.
Further limitations associated with measuring a transmitted beam are related to the position of objects along the beam path. Positional information of light attenuating objects is difficult to obtain because only the transmitted beam intensity is measured. If two objects obscure the beam, permitting only a weak beam to be transmitted, it is difficult to distinguish between them. Reversing the positions of the source and detector generates the same signal because the attenuating path remains the same. It is thus difficult to discern between one, two or more objects in the beam path.